Prefilled syringes which are preliminarily filled with drug solution are known. The prefilled syringe includes an outer cylinder provided with a discharge port at the distal end thereof, a gasket inserted in the outer cylinder, and a plunger (pushing element) connected to the gasket, with the drug solution contained in the space surrounded by the outer cylinder and the gasket.
At the time of injecting a drug solution into a living body by use of the prefilled syringe, a needle assembly having a hollow needle is mounted to the discharge port of the outer cylinder, and the prefilled syringe is used in this mounted state (refer to, for example, Patent Document 1). Further, in the mounted state, the inside of the outer cylinder and the inside of the hollow needle communicate with each other. The needle assembly (protective sleeve) described in Patent Document 1 includes a hollow needle, a hollow cylindrical outside member (outside sleeve member) arranged on the outer peripheral side of the hollow needle and supporting and fixing a proximal portion of the hollow needle, a hollow cylindrical inside member (inside sleeve member) disposed between the outside member and the hollow needle concentrically with the outside member and movable along the axial direction of the outside member, and a coil spring for biasing the inside member in the distal direction. In the needle assembly thus configured, the inside member may be positioned in a first state of covering the needle body up to the needlepoint or a second state of exposing the needlepoint by moving relative to the outside member along the axial direction of the outside member. In addition, by the biasing force of the coil spring, the inside member can be maintained in the first state.
In the needle assembly described in Patent Document 1, however, the inside member in the first state is brought into the second state when it is only pushed in the proximal direction against the biasing force of the coil spring. Therefore, even in the case where such a pushing force is inadvertently exerted on the inside member, the inside member is put into the second state, resulting in that a finger or the like may be punctured, by mistake, by the needlepoint exposed from the inside member. Further, in the needle assembly described in Patent Document 1, regardless of whether the needlepoint protrudes from the inside member, the drug solution would flow out through the hollow needle if only the plunger is pushed. Therefore, in the case where the plunger is pushed inadvertently, the drug solution would flow out uselessly (would be wasted), making it impossible to inject a predetermined amount of drug solution.    Patent Document 1: Japanese Patent No. 2872318